chore: init @launchdarkly/client-testing-plugin

[joker23]

This PR pushes the initial implementation of the client test data plugin

SDK-2124

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Note

Medium Risk
Introduces a new publishable workspace package that hooks into the SDK debug-override path; risk is mainly around correctness/compatibility of the plugin interface and its interaction with client initialization and change events.

Overview
Adds a new workspace package, @launchdarkly/client-testing-plugin, providing a TestData plugin that injects and dynamically updates client-side flag values via the SDK’s debug-override mechanism (including typed setters, remove, clear, and primitive-update deduping).

Includes package build/test tooling (tsup, tsconfig references, Jest + jsdom setup) plus unit and browser e2e tests validating real SDK integration and change event behavior. Repo wiring is updated to include the workspace and TS project reference, and .gitignore now ignores .claude/stacks/.

^{Reviewed by Cursor Bugbot for commit b9eed76. Bugbot is set up for automated code reviews on this repo. Configure here.}

[joker23]

@cursor review

[github-actions]

@launchdarkly/js-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 26281 bytes
Compressed size limit: 29000
Uncompressed size: 128971 bytes

[github-actions]

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
Compressed size: 31906 bytes
Compressed size limit: 34000
Uncompressed size: 113658 bytes

[github-actions]

@launchdarkly/browser size report
This is the brotli compressed size of the ESM build.
Compressed size: 179498 bytes
Compressed size limit: 200000
Uncompressed size: 830837 bytes

[github-actions]

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 38487 bytes
Compressed size limit: 39000
Uncompressed size: 211236 bytes

[joker23]

@cursor review

[joker23]

@cursor review

[joker23]

@cursor review

[joker23]

@cursor review

[cursor]

✅ Bugbot reviewed your changes and found no new issues!

Comment @cursor review or bugbot run to trigger another review on this PR

^{Reviewed by Cursor Bugbot for commit b9eed76. Configure here.}

[devin-ai-integration]

Devin Review found 1 potential issue.

View 5 additional findings in Devin Review.

[devin-ai-integration]

🟡 describe block used without shared configuration, violating testing guidelines

The describe('TestData', ...) block in TestData.test.ts wraps all tests but has no shared setup — no beforeEach, no afterEach, no shared let variables. Every test creates its own TestData instance and mock independently.

CLAUDE.md and .cursor/rules/testing.mdc both explicitly state: "Describe blocks should ONLY be used for common setup for a series of tests" and "If there is no shared configuration, do not use a describe block." The existing analogous test file at packages/shared/sdk-client/__tests__/plugins/safeRegisterDebugOverridePlugins.test.ts correctly uses top-level it(...) blocks without a wrapping describe.

Prompt for agents

Remove the wrapping describe('TestData', () => { ... }) block from TestData.test.ts since there is no shared configuration (no beforeEach, afterEach, or shared variables). All it(...) test cases should be promoted to the top level of the file, and the closing }); of the describe block at the end of the file should be removed. This matches the pattern used in packages/shared/sdk-client/__tests__/plugins/safeRegisterDebugOverridePlugins.test.ts which has similar standalone tests without a describe wrapper.

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